FIG. 1 shows a dishwasher of the kind to which the present invention relates. It has a container 1 with withdrawable baskets 2 and 3 for the articles to be washed up. A typical use of a machine of this kind is as household dishwasher, whereby the articles to be washed up normally consist of plates, glasses, cutlery, pots, and the like. Below the upper basket 2 an upper spray arm 4 is arranged, which is supplied with flushing water through the pipe 11 during the washing up. The dish-water is sprayed out through a number of spray nozzles 41 on the arm, at least some of these spray nozzles being inclined in order to impart rotation to the spray arm during the flushing. The spray arm is journalled to be easily movable on a stub tube 12 serving as a hub, through which stub tube the flushing water from the pipe 11 flows out into the spray arm through openings 14. In similar manner, the spray arm 5 mounted below the lower basket 3 is mounted to be easily movable on a stub tube 10 serving as a hub, the arm is supplied with dishwater during the flushing through openings 13, and the dishwater flows out through a number of spray nozzles 51 on the arm. The dishwater sprayed out through the nozzles of the spray arms during the flushing runs down into the lower part of the container and is collected in a sump 6. The flushing system includes a circulation pump 8. The pump 8 sucks from the sump 6 through a pipe 7 and pumps the dish water via the pipes 9 and 11 to the two spray arms. To prevent resoiling of the items being washed up, the flushing system is provided with a strainer 15. The strainer 15 has an upper strainer surface slightly inclined towards the center of the strainer as well as a stub tube 152 arranged centrally in the strainer and extending down to the bottom of the sump 6. Both the upper strainer surface 151 and the stub tube 152 are constructed as strainers, for example made of sheet metal and provided with a large number of fine holes.
In the bottom of the sump 6 and below the stub tube 152 of the fine strainer, a waste pipe 16 is arranged through which the dishwater can be drained off with the aid of a discharge pump 17 and an outlet pipe 18. To prevent coarser particles from causing clogging of the discharge pump 17 or the waste pipe 16, a coarse strainer 19 is arranged at the outlet from the dishwasher.
Dish water is supplied to the machine through a supply pipe 20, a controlled movement downwards into the container 1 being imparted to the dishwater by a shield 23. For control of the water supply a solenoid valve 21 is arranged in the supply pipe. The function of the dishwasher is controlled and monitored by a control means 24. In conventional manner, this may consist either of a conventional, rotary electromechanical control means, or of a microprocessor or the like which is provided with suitable input and output means. To start the washing up, select the desired program, and the like, a number of schematically shown operating members 25, 26, 27 are arranged.
A typical water level during any of the washing steps is shown by the horizontal line A in FIG. 1.
The control means 24 controls the washing procedure in a plurality of consecutive washing steps, for example pre-wash, main washing and three rinsings. Before the pre-wash a flushing step may possibly be provided, between the pre-wash and the main washing an intermediate rinsing step may be provided, and after the last rinsing a drying step may be provided during which the washed articles are dried with the aid of heating elements activated by the control means and built into the dishwasher. In addition, the heating elements may be used for heating the admitted dish-water to the desired temperature at the beginning of each washing step.
FIG. 2 shows in more detail the strainer system of the dishwasher as well as the lower spray arm 5. The flushing water is supplied to the spray arm through the stub tube 10 and the openings 13 provided therein. The spray arm rests on a flange 102 on the stub tube 10 and is readily movable around the stub tube. The spray arm has, for example, the three spray nozzles 51a, 51b and 51c, arranged on the upper side, as well as an additional nozzle 51d on the underside. The figure shows the fine strainer 15 with its upper strainer surface 151, and the stub tube 152. These two elements may be made of sheet metal and are provided with a large number of relatively fine holes 153, 154. In the lefthand part of the sump 6 a stub tube 71 is provided for connection of the pipe or hose 7 to the circulation pump 8. Similarly, in its central part the sump has a stub tube 161 for connection of the pipe or hose 16 leading to the discharge pump 17. The coarse strainer 19, which is to capture such larger objects that may cause a pump stop, is designed as a coarse-meshed metal or plastic strainer with holes 191 and is suspended from a flange in the stub tube 152 of the fine strainer.
During, for example, the prewash, normally large quantities of dirt are removed from the articles to be washed up. The dirt accompanies the water flowing down into the bottom of the container 1 and remains on the surface of the fine strainer 15, to which it adheres due to the powerful suction action of the circulation pump 8. The soiling is particularly pronounced on the upper strainer surface 151. In known dishwasher a prewash is terminated by the flushing ceasing (the circulation pump 8 being stopped), whereafter the dishwasher is pumped out with the aid of the discharge pump 17. Part of the dirt deposited on the surface of the fine strainer is discharged with the dishwasher whereas part of the dirt remains on the strainer surface. During a subsequent wash-up step, for example the main wash-up succeeding the prewash, this remaining dirt will soil the dishwasher and cause resoiling of the articles being washing. This resoiling requires a relatively large number of rinsings after the main washing, for example three such rinsings. The consumption of water for a complete washing procedure is therefore relatively great. This is a considerable disadvantage since both the water costs and the cost of heating the water are relatively high.
Proposals have been made to introduce extra rinsings after the prewash to rinse away, as far as possible, the dirt removed during the prewash, thus reducing the resoiling during the main washing. However, no reduction of the total number of necessary rinsings and hence of the total water consumption can be obtained in this way. Further, an obvious method of reducing the water consumption would be to reduce the volume of water used during each rinsing. However, it has proved to be difficult to reduce the water consumption in this way, since a volume of water reduced below a certain limit results in the circulation pump sucking air, which leads to a deteriorated flushing action and an increased noise level. Similarly, it has proved to be impossible to reduce the water consumption by reducing the number of rinsings without simultaneously deteriorating the desired good result of the washing.
From DE-A1-2 657 764 a dishwasher of the kind described above is previously known, in which, during the pumping out of washingup liquid after a washing step, the circulation pump is kept running while the liquid is continuously pumped out at least until the liquid level has dropped to below the upper strainer surface of the fine strainer. This is done for the purpose of obtaining a rinsing of the fine strainer. However, tests performed have shown that, in a typical dishwasher, no significant improvement is achieved in this way. No real cleaning of the strainer surface is obtained until, during the pumping out, the liquid surface has dropped to such an extent that the strainer surface is exposed. The remaining quantity of liquid is then so small that the further pumping out is performed very rapidly, and no significant cleaning of the filter surface is ever performed.
EP-A1-222 306 discloses a known dishwasher in which the spray arm has nozzles on its underside which provide a flow of water in the washing liquid with a component parallel to the surface of the fine strainer. In dishwashers of this kind, the liquid level during the wash-up steps is so high that the liquid surface lies above the strainer surface. The proposed method therefore only provides a certain amount of flow in the washing liquid along the strainer surface and only incomplete cleaning of the strainer. The circulation pump working during the wash-up steps has a high capacity and a powerful sucking action. This causes particles of dirt to adhere to the surface of the strainer, which further prevents an efficient cleaning of the strainer.